1struct User {
2    let id: Int
3    var name: String
4    var email: String?       // Optional — defaults to nil
5    var avatarURL: String?   // Optional — defaults to nil
6}
7
8// Full initialization — providing all properties
9let user1 = User(id: 1, name: "Alice", email: "[email protected]", avatarURL: nil)
10
11// Omitting optional properties — they default to nil
12let user2 = User(id: 2, name: "Bob")
13
14// Providing some optionals
15let user3 = User(id: 3, name: "Charlie", email: "[email protected]")
16
17print(user2.email)  // nil
18print(user3.email)  // Optional("[email protected]")

1struct Configuration {
2    var host: String = "localhost"
3    var port: Int = 8080
4    var timeout: Double? = nil
5    var maxRetries: Int? = 3       // Optional with a non-nil default
6    var apiKey: String?            // Optional, defaults to nil
7}
8
9// Use all defaults
10let config1 = Configuration()
11
12// Override specific values
13let config2 = Configuration(host: "api.example.com", port: 443)
14
15// Override an optional
16let config3 = Configuration(timeout: 30.0, maxRetries: 5)
17
18print(config1.host)        // "localhost"
19print(config1.maxRetries)  // Optional(3)
20print(config1.apiKey)      // nil

1struct Rectangle {
2    var width: Double
3    var height: Double
4    var color: String?
5    var label: String?
6
7    // Custom initializer for square
8    init(side: Double, color: String? = nil) {
9        self.width = side
10        self.height = side
11        self.color = color
12        self.label = nil
13    }
14
15    // Custom initializer with validation
16    init?(width: Double, height: Double) {
17        guard width > 0, height > 0 else { return nil }
18        self.width = width
19        self.height = height
20        self.color = nil
21        self.label = nil
22    }
23}
24
25let square = Rectangle(side: 5.0, color: "red")
26let rect = Rectangle(width: 10, height: 20)  // Returns Optional

1struct Profile {
2    var username: String
3    var bio: String?
4    var website: String?
5}
6
7// Define custom initializers in an extension to keep the memberwise init
8extension Profile {
9    init(username: String) {
10        self.username = username
11        self.bio = nil
12        self.website = nil
13    }
14
15    init(from dictionary: [String: String]) {
16        self.username = dictionary["username"] ?? "unknown"
17        self.bio = dictionary["bio"]
18        self.website = dictionary["website"]
19    }
20}
21
22// Memberwise init still works
23let p1 = Profile(username: "alice", bio: "Developer", website: "https://alice.dev")
24
25// Custom inits also work
26let p2 = Profile(username: "bob")
27let p3 = Profile(from: ["username": "charlie", "bio": "Designer"])

1struct APIResponse: Codable {
2    let id: Int
3    let title: String
4    var subtitle: String?    // Missing keys decode as nil
5    var imageURL: String?    // Missing keys decode as nil
6    var tags: [String]?      // Missing keys decode as nil
7}
8
9let json = """
10{"id": 1, "title": "Hello World"}
11""".data(using: .utf8)!
12
13let response = try JSONDecoder().decode(APIResponse.self, from: json)
14print(response.title)     // "Hello World"
15print(response.subtitle)  // nil — key was missing in JSON
16print(response.tags)      // nil

1struct Task {
2    let id: Int
3    var title: String
4    var completedAt: Date?
5    var assignee: String?
6
7    mutating func complete() {
8        completedAt = Date()
9    }
10
11    mutating func assign(to person: String) {
12        assignee = person
13    }
14
15    mutating func unassign() {
16        assignee = nil  // Set optional back to nil
17    }
18}
19
20var task = Task(id: 1, title: "Fix bug")
21print(task.completedAt)  // nil
22
23task.assign(to: "Alice")
24print(task.assignee)     // Optional("Alice")
25
26task.complete()
27print(task.completedAt)  // Optional(2025-03-15 ...)